1. Field of the Invention
This invention relates to a reduction gear for use in a seat of a vehicle for reducing the rotating speed of a drive shaft connected to a prime mover and transmitting its rotary motion to an output shaft.
2. Discussion of the Related Art
Shown in FIG. 9 is a conventionally known reduction gear 100 suited for use in a power seat of a vehicle, for example. This reduction gear 100 comprises as its basic constituent elements a worm 103 formed integrally with an inner shaft 103a which is coaxially connected to the end of a drive shaft 102 of a driving motor 101, a worm wheel 104 engaged with the worm 103, an output shaft 105 combined integrally with the worm wheel 104 which is formed concentrically around the curved outer surface of the output shaft 105, and a casing 106 in which the worm 103 and the worm wheel 104 are fitted.
The casing 106 is provided with a tubular housing 107 accommodating the worm 103, and a cylindrical bearing plug 108 for rotatably supporting the extreme end of the drive shaft 102 is fitted into a terminal end (right end in FIG. 9) of the tubular housing 107. There is formed a shaft-supporting hole 109 in an inner end surface of the cylindrical bearing plug 108 on a common axis with the drive shaft 102. The inner diameter of the shaft-supporting hole 109 is made slightly larger than the outer diameter of an outer shaft 103b of the worm 103 so that the extreme end of the worm 103 is rotatably fitted into the shaft-supporting hole 109.
Further, a small hole is made in an end surface of the outer shaft 103b, and a steel ball 110 is fitted concentrically in the small hole with part of the steel ball 110 protruding outward from the small hole. On the other hand, a washer 111 is fitted at the bottom of the shaft-supporting hole 109, and the worm 103 is allowed to rotate smoothly with the washer 111 held in contact with the steel ball 110.
In the reduction gear 100 thus constructed, the rotary motion of the driving motor 101 is transmitted to the worm 103, and the rotation of the worm 103 is transmitted to the worm wheel 104, causing the worm wheel 104 to turn at a reduced rotating speed. Since the rotating speed of the worm wheel 104 is reduced, the output shaft 105 turns at a lower speed than the drive shaft 102.
In the aforementioned conventional reduction gear 100, variations in the length of the worm 103 are compensated by adjusting the length of thread engagement of the cylindrical bearing plug 108 and by elastic deformation of the washer 111 which is in contact with the steel ball 110. However, the amount of elastic deformation of the washer 111 could be smaller than the amount of variations in the length of the worm 103 and, thus, the reduction gear 100 has a problem that it might not be able to fully compensate for dimensional variations of the worm 103. It is therefore necessary to improve the accuracy of the length of the worm 103 and this would result in an increase in manufacturing cost.
Further, the length of thread engagement between the cylindrical bearing plug 108 and the tubular housing 107 is apt to vary due to vibrations or other causes. Therefore, the cylindrical bearing plug 108 could loosen in a long run, causing the steel ball 110 at the extreme end surface of the worm 103 to be separated from the washer 111, even though the end of the outer shaft 103b of the worm 103 is securely supported by the cylindrical bearing plug 108 immediately after assembly. Thus, there arises a problem that the worm 103 would become shaky in its axial direction. When this has happened, it is necessary to readjust the thread engagement of the cylindrical bearing plug 108 by tightening it, which is quite inconvenient for users.
Furthermore, since three components including the cylindrical bearing plug 108, the steel ball 110 and the washer 111 are required for rotatably supporting the extreme end of the worm 103, the conventional reduction gear 100 entails high component costs and complex assembly operation. This poses another problem that the efficiency of assembly work is relatively low.
1. Field of the Invention
It is an object of the invention to provide a reduction gear for a vehicle seat which has overcome the aforementioned problems in the prior art.
According to an aspect of the invention, a reduction gear for a vehicle seat, comprises a worm connectable with an end of a drive shaft of a driver, a worm wheel meshed with the worm and connectable with an output shaft, and a casing for rotatably supporting the worm and the worm wheel. The casing is formed with a hollow cylindrical portion for accommodating the worm, and provided with a resilient member for urging the worm toward the drive shaft.
Other features of the present invention will be more readily understood from the following detailed description when read in conjunction with accompanying drawings.